Orochi Warriors is a great fan service; combining the both Koei Warriors, Koei have made a game that no fan can resist.
If you aren´t a fan of the Koei Warriors, you should really try to avoid this game, because it´s pure fan service. But if you are a fan, this is your game. It lacks of new ideas, expect the one which makes you able to take any three characters to form your party, but that shouldn´t make any difference for the fans.
If you are a fan whose looking for challenges, you´re going to disappoint. This game is extremely easy. If you take damage, you can simply change your character and let the damaged one to rest. The overall difficulty level is easy if you have ever played the Koei Warriors before. If you are a newbie with the series, this game is the game where you should start playing the series. But beware of the story that´s very odd.
You are going to be asking why did I gave 9 for this game? As I said before this game is for the fans. And I am a fan. It´s just that simple.